The preparation of phosphate fertilizers by means of alkaline high temperature decomposition of naturally occurring calcium phosphates is well known in the art. For example, according to the German Patent No. 481,177, alkali carbonate and silicic acid are used as decomposing agents, and this mixture of natural crude phosphates, alkali carbonate and silicic acid is heated to a calcining temperature of above 1100.degree. C. in a rotary kiln. During such a process, the formation of alkali calcium phosphate from tricalcium phosphate is presumably taking place in a reaction represented by the following empirical equation: EQU 2Ca.sub.3 (PO.sub.4).sub.2 +SiO.sub.2 +2Na.sub.2 CO.sub.3 .fwdarw.2(Na.sub.2 O.2CaO.P.sub.2 O.sub.5)+2CaO.SiO.sub.2 +2CO.sub.2
according to this equation, at least 1 mole of Na.sub.2 O is present per 1 mole of P.sub.2 O.sub.5, and the added amount of SiO.sub.2 is adjusted so that the CaO which is not bound to the P.sub.2 O.sub.5 in a molar ratio of 2:1 is transformed into calcium orthosilicate.
Later it has been found that it is advantageous if the molar ratio Na.sub.2 O/P.sub.2 O.sub.5 is between 1.1:1 and 1.8:1 and the decomposition is effected in the presence of water vapor which can be provided by using fuels, in particular fuel oils, which are rich in hydrogen.
The phosphate component of the calcium-sodium-silicophosphate which has been industrially produced in this manner for more than 50 years is plant-available on practically all types of cultivated soils and exhibits a high P.sub.2 O.sub.5 solubility in standard test solutions, e.g. in a test solution according to Petermann, in a neutral solution of ammonium citrate, and in a 2% citric acid solution.
More recently, in addition to soda, aqueous solutions of alkali hydroxides have also been used industrially as decomposition agents, as is disclosed in German Pat. No. 1,592,690 and No. 2,128,133 and the corresponding U.S. Pat. Nos. 3,713,803 and 3,985,537. By using a potassium hydroxide solution, potassium containing calcined phosphate fertilizers can be obtained. The latter which contain about 50% of P.sub.2 O.sub.5 +K.sub.2 O are valuable multi-nutrient fertilizers.
Depending on the type of the naturally occurring raw phosphates and the alkali compounds which are used, the phosphates which can be produced in a calcination process exhibit a P.sub.2 O.sub.5 content of from about 20 to about 30%.
However, over the course of time, consumers have come to desire an increasingly higher nutrient content in a calcined phosphate product obtained with the aid of soda or a sodium hydroxide solution. Therefore, only calcined phosphates including a high percentage of P.sub.2 O.sub.5 can be sold any more. Since, on the one hand, high percentage types of raw phosphates are becoming less and less available for producing a phosphate product of a constant composition, containing for example more than 29% of P.sub.2 O.sub.5, and on the other hand, raw phosphates including a lower percentage of P.sub.2 O.sub.5 are available at low costs throughout the world, the problem of how to concentrate the calcined phosphates directly or indirectly in an economical procedure has been present for many years already. In spite of continuous efforts, no satisfying solution of this problem has been found so far.
In all processes which so far have been proposed for directly increasing the P.sub.2 O.sub.5 content in phosphate fertilizers, the additional P.sub.2 O.sub.5 is provided by adding alkali phosphates to the raw phosphate prior to introducing the mixture into the rotary kiln. Further to the usually coarse grained crystalline orthophosphates, less exactly defined alkali phosphates are also used which have been prepared from phosphoric acid and alkali metal compounds shortly before their use.
Thus, German Pat. No. 681,699 discloses the preparation of alkali-calcium phosphates wherein the P.sub.2 O.sub.5 content is above 38% by calcining raw mineral phosphates together with trialkali orthophosphates at about 1000.degree.-1100.degree. C. The added amount of trialkali orthophosphate is preferably such that about 0.8 moles of P.sub.2 O.sub.5 are provided by the latter per mole of P.sub.2 O.sub.5 in the raw mineral phosphates. In this patent it is also mentioned that, instead of the trialkali orthophosphate, mixtures of salts which are chemical precursors of this orthophosphate salt can be used; for example, mixtures of sodium metaphosphate and soda, of monosodium orthophosphate and soda, or of sodium pyrophosphate and soda. Furthermore, German Pat. No. 721,412 discloses a process for preparing phosphate fertilizers wherein the raw phosphate is mixed with such an amount of dialkali phosphates, e.g., disodium phosphate, which provides 0.8 to 1.5 moles of P.sub.2 O.sub.5 per 1 mole of P.sub.2 O.sub.5 in the raw phosphate, and wherein calcining temperatures up to about 1150.degree. C. are used. The addition of such solid alkali phosphates has the disadvantage that these processes cannot be carried out in a rotary kiln on an industrial scale. Deposits on the wall of the kiln immediately occur after introducing the mixture into the rotary kiln. Such deposits can lead to a plugging of the kiln and can cause the formation of products of varying compositions.
It has further been attempted to avoid the use of valuable solid alkali phosphates by adding the necessary amounts of aqueous phosphoric acid and soda to the raw phoshpate prior to the introduction into the kiln (see, e.g., German Pat. No. 729,909). Yet hereby also, similar difficulties as with the use of solid alkali phosphates are encountered in practice.
Repeated disturbances which can vary from short interruptions to a complete standstill of kiln operation affect the calcining process in the rotary kiln. Furthermore, the P.sub.2 O.sub.5 solubility of the resulting reaction products in the Petermann test solution is inconsistent. These differences in solubility can be very high.
According to other recent proposals, the addition of silicic acid can be omitted completely or partly, if a correspondingly increased amount of phosphoric acid is added. The molar ratio P.sub.2 O.sub.5 /CaO/K.sub.2 O within the mixture which is to be calcined should preferably be about 1:2-3:1.3-1.7 whereby the amount of P.sub.2 O.sub.5 which is derived from the added phosphoric acid can comprise between 0.2 and 1.6 moles per mole of P.sub.2 O.sub.5 from the phosphate mineral.
For preparing the mixture which is to be calcined, the crude phosphate is mixed with phosphoric acid and potassium hydroxide in such a way that potassium orthophosphate can be formed from the latter two components prior to introducing the mixture into the rotary kiln. In this procedure also, the above described difficulties occur during the passing of the mixture through the kiln. Even if the mixture is additionally granulated, these difficulties cannot completely be overcome. Therefore, it has already been proposed to use a tunnel furnace instead of a rotary kiln for the calcination process. Yet this would mean giving up all the technical advantages which are connected with carrying out the reaction in a rotary kiln.